This invention relates generally to the production of pellets of uniform size from a body of hot flowable polymeric composition material, facilitating storage, packaging, transporting, weighing and other uses in the plastics industry. More particularly, the invention relates to a method and apparatus for cooling the pellets immediately after they are cut from the extruder to prevent adhesion between pellets, and undesired agglomeration.
The common practice is to extrude and cut the polymer into pellets when it is in a liquid state at an elevated temperature, referred to for brevity as a molten state. The pellets are then collected and cooled to a solid state by any one of a variety of methods and apparatus. Pelletizers such as that described in Mattera U.S. Pat. No. 5,215,763 comprise a number of rotating knives which cut strands of the polymer as they are extruded from a die face, impelling the hot pellets in air and into a cooling fluid, usually water. The water is arranged to flow over surfaces that surround the die knife area. If the pellets have a specific gravity greater than one they usually become enveloped by the water, and all surfaces are subjected to cooling at about the same time and at the same rate of cooling. Under properly adjusted conditions the pellets usually stay separated from one another and do not adhere together or agglomerate. However, in some cases there is a tendency for pellets to float in the water either because they have a specific gravity less than one, or because water vapor bubbles form locally and adhere to their surfaces. Pellets that float tend to be cooled only on the surfaces immersed in water, while other surfaces retain a higher temperature for a longer period of time. When such pellets become concentrated at the surface rather than being distributed throughout the water, agitation causes them to come into mutual contact and to adhere at surfaces that have not been sufficiently cooled. This leads to nonuniformity in the sizes of the resulting solids and consequent degradation of the product.
The principal object of this invention is to provide a method and apparatus for producing polymer pellets which controls the flow of cooling fluid so that it arrests, envelopes and controls the movement of the pellets so that, whether having a specific gravity greater than or less than one, they are caused to be uniformly exposed to the cooling fluid on all surfaces thereof. Thereby, the uniform cooling of each pellet reduces the probability of sticking to another pellet as the polymer is being cooled down to the solid, nonsticking state.
With the foregoing object in view, this invention includes a method and apparatus whereby a pellet slurry, comprising a stream of cooling fluid and polymer pellets cut from a hot die face and quickly immersed therein, is quickly introduced into a series of sluices which cause the slurry to be repeatedly reversed in direction and simultaneously inverted, causing the floating pellets to be repeatedly moved toward the bottom of and fully immersed in the flowing fluid, thereby enveloping, cooling and buffering the whole periphery of each pellet with fluid.
The pellets are initially enveloped in the fluid in a suitable pellet forming apparatus, and the pellet laden slurry is discharged into a columnar collector. The pellet slurry falls in a free trajectory until it impacts the surface of a cascade array of baffles. Each baffle directs the stream to flow in a horizontal and vertically descending path. While flowing in such path, the lighter pellets begin to float and segregate at the top of the fluid stream. The stream is then deflected and inverted to reverse its horizontal direction, causing it to flow in another horizontal and vertically descending path. At each repeated inversion of the stream the portion of the slurry at the top of the stream, viewed in a vertical cross section, is reversed to the bottom. Thus the floating pellets are each repeatedly immersed in the fluid during the time period required for cooling to a desired temperature below which sticking and agglomeration do not occur. The floating and reimmersion continues through successive baffle reversals until the pellets are cool enough. The pellet slurry is then usually discharged from the columnar collector into a dryer.